LEDs convert electrical energy into optical energy. In semiconductor LEDs, light is usually generated through recombination of electrons, originating from an n-type doped semiconductor layer, and holes originating from a p-type doped semiconductor layer. In some infra-red emitting semiconductor materials light can be generated by electron intersub-band transitions rather than electron hole transitions. Herein, the area where the main light generation takes place is termed the light-emitting layer.
Further, as used herein, the term “light” is used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
A major challenge is to extract as much of the emitted light as possible from the semiconductor material into the surrounding medium, typically air. This is hindered by total internal reflection at the surfaces of the semiconductor.
In traditional cuboid shaped LED devices, the average path length for light rays within the semiconductor is long, and the average number of reflections of an emitted light ray at semiconductor surfaces is high, prior to escape from the device. Long path lengths and reflections at metal coated semiconductor surfaces both lead to absorption losses. The light that does escape, escapes to a large extent through the sides of the chip and an external mirror may be used to collect this light into a useful light beam. Another approach is called chip shaping. Higher extraction efficiencies (EE) are possible with this approach. However, it does not eliminate the long path lengths within the semiconductor chip, nor the requirement for an external mirror. Also, the technique is less suitable to the widely used gallium nitride (GaN) based materials systems. The reason for this is that the sapphire and silicon carbide (SiC) substrates commonly used in GaN-based LED-chips are both very hard materials and very difficult to shape mechanically, for example with a dicing saw. In LED devices using such materials, it seems not to be a practical solution to shape the whole chip.
As used herein, the term “extraction efficiency” encompasses the amount of useful light extracted from an LED device as a proportion of the total light generated by the device. The EE may be expressed as a percentage.
Another common approach to improve the EE of LEDs is to roughen the surfaces where the light exits the chip. This reduces the amount of light trapped by total internal reflection that occurs by randomising the angles at which the light hits the surface.
LEDs have a wide range of applications. They are used in displays, medical devices, diagnostic devices, vision equipment, projectors and consumer goods amongst other things. In a large number of these applications the light generated by the LED must be further manipulated after it exits the chip. For example in a projector the light generated should be focused on a screen which is to be viewed. This typically requires the positioning of lenses, reflectors and/or other components between the source and the screen. In another example, when used in a fluorescence based analysis system the light generated should be focussed on to the sample of interest, and may require optical filtering and mitigation of the amount of stray light.
Therefore, additional components are typically required for the correct operation of an LED device, but they add bulk and complexity to the device itself and the optical path of the light within the device before it escapes from the light emitting surface. These issues become of greater importance when the total system or the object of interest is small.
U.S. Pat. No. 7,518,149 describes an LED device with improved the EE and producing light which exits the LED chip within a defined set of angles. The device described in U.S. Pat. No. 7,518,149 is referred to as a μLED and is created as an integrated diode structure in a mesa, in which the mesa shape and the light-emitting region are chosen for optimum EE.